About.com’s Amanda Briney has a primer on great circles. A great circle is the shortest distance between two points on a sphere; sailors and aviators use great circles to get the fastest and most efficient route from point A to point B (though air and ocean currents change the route in practice). On a gnomonic projection, a great circle is a straight line; on a rectangular projection like the Mercator, a great circles appears as a curve. Now you know why trans-Atlantic flights always seem to veer north and pass over Greenland: it’s shorter. Via About.com Geography.